Method and apparatus for improving stability of a 6T CMOS SRAM cell

ABSTRACT

The present invention is a CMOS SRAM cell comprising two access devices, each access device comprised of a tri-gate transistor having a single fin; two pull-up devices, each pull-up device comprised of a tri-gate transistor having a single fin; and two pull-down devices, each pull-down device comprised of a tri-gate transistor having multiple fins. A method for manufacturing the CMOS SRAM cell, including the dual fin tri-gate transistor is also provided.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/165,724, filed on Jun. 24, 2005, which is currently pending, whichclaims the benefit of U.S. patent application Ser. No. 10/679,124, nowU.S. Pat. No. 6,970,373.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit design and manufacturing, and more specifically to a 6T CMOSSRAM cell using tri-gate fully depleted substrate transistors and itsmethods of fabrication.

2. Discussion of Related Art

As silicon technology continues to scale from one generation to thenext, the impact of intrinsic threshold voltage (Vt) variations inminimum geometry size bulk planar transistors reduces the CMOS SRAM cellstatic noise margin (SNM). This reduction in SNM caused by increasinglysmaller transistor geometries is undesirable. SNM is further reducedwhen Vcc is scaled to a lower voltage.

Threshold voltage (Vt) variations in planar transistors arise mostlyfrom the statistical fluctuation of the number and/or location of thedopant atoms in the depletion region of the transistors. The Vtvariations pose barriers to the scaling of supply voltage, transistorsize, and, hence, the minimum six transistor (6T) CMOS SRAM cell size.This limits the total transistor count for conventional 6TSRAM-dominated high performance CMOS ASICs and microprocessors due todie size and cost constraints.

Currently, the problem of reduced SNM resulting from Vt instability ofthe SRAM cell transistors is solved at the circuit/layout level byeither (a) increasing the minimum supply voltage (Vccmin) needed tooperate the cell and keeping the minimum geometry size transistors or(b) increasing the channel length and width of the cell transistors toenable a lower minimum operating voltage at the expense of the minimumcell size. At the device level, in planar devices, Vt mismatch arisingfrom random dopant fluctuations (RDF) can be minimized by box-shapedwells or super-steep retrograde wells at the expense of additionalfabrication process complexity.

A 6T CMOS SRAM cell circuit diagram using planar transistors isillustrated in FIG. 1. The SRAM cell consists of two N-type accessdevices 102, two N-type pull-down devices 104, and two P-type pull-updevices 106.

FIG. 2 illustrates a 6T CMOS SRAM cell layout using planar transistors.The gate of each access device is located in region 202. The gate ofeach pull-down device is located in region 204. The gate of each pull-updevice is located in region 206. The gate regions are indicated by aregion of polysilicon 214 over a region of P-type diffusion 212 orN-type diffusion 210. Metal layers 218 provide power (Vcc) and ground(Vss). Metal layers 218 may also connect the gate/source/drain of oneplanar transistor in the cell to the gate/source/drain of anothertransistor in the cell, and may interconnect one cell to another.Contacts 216 indicate regions where connections may be made to the metallayers. For a given Vcc, the cell ratio is tailored by sizing eachaccess transistor width and each pull-down transistor width to achievethe maximum SNM value.

FIG. 3 is a graph 300 which illustrates the impact of supply voltagescaling on a typical 6T CMOS SRAM cell using planar transistors. Thenoise margin values assume nominal threshold voltage, nominal Vcc, andnominal device sizes. Dashed line 310 indicates the minimum desiredvalue for SNM, 240 mV. The graph shows that as Vcc scales down from 2Vto less than 1V, the cell ratio must increase in order to maintain adesirable SNM value. For a cell ratio of 1.5 (302), the minimum voltagethat can be achieved while maintaining a nominal SNM of 240 mV isslightly less than 2.0V. When the cell ratio is increased to 2.0 (304),the minimum voltage that can be achieved while maintaining a nominal SNMis less than 1.5V. If the cell ratio is increased to 3.5 (306), theminimum voltage may be reduced to less than 1.0V. However, increasingthe cell ratio corresponds to an area penalty in the form of increasingcell size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a prior art 6T CMOS SRAM cell circuitdiagram using planar transistors.

FIG. 2 is an illustration of a prior art 6T CMOS SRAM cell layout usingplanar transistors.

FIG. 3 is a graph which illustrates Static Noise Margin as a function ofSupply Voltage for a 6T SRAM cell of varying cell ratios.

FIG. 4 is a cross-sectional view of a single-fin tri-gate transistor.

FIG. 5 is a cross-sectional view of a dual fin tri-gate transistor.

FIG. 6 is an illustration of a 6T CMOS SRAM cell circuit diagram withtri-gate transistors according to one embodiment of the presentinvention.

FIG. 7 is an illustration of a 6T CMOS SRAM cell layout using single anddual fin tri-gate transistor according to one embodiment of the presentinvention.

FIG. 8 is a comparison of the gate width of a dual fin tri-gatetransistor according to one embodiment of the present invention and aplanar transistor.

FIG. 9 is a graph which compares SNM as a function of supply voltage fora tri-gate SRAM according to one embodiment of the present invention andfor a planar SRAM, where both SRAM cells have the same layout area.

FIG. 10 is a flowchart which describes the steps in forming a dual fintri-gate transistor according to one embodiment of the presentinvention.

FIGS. 11A-11J illustrate the formation of a dual fin tri-gate transistoraccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention is a 6T CMOS SRAM cell using non-planar tri-gatetransistors, and its method of fabrication. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. In other instances,well-known semiconductor process and manufacturing techniques have notbeen described in particular detail in order to not unnecessarilyobscure the present invention.

The present invention takes advantage of the higher drive currentperformance of non-planar tri-gate transistors to improve the stabilityof the 6T CMOS SRAM cell, thus enabling lower supply voltage operationand reduced cell layout sizes. Tri-gate transistors in multiple finconfigurations can deliver more drive current for a given layout widththan a planar transistor.

FIG. 4 illustrates a cross section of a typical single fin tri-gatetransistor 400. A single fin tri-gate transistor is a tri-gatetransistor having a single semiconductor body 410. The semiconductorbody will also be referred to as a “semiconductor fin.” Thesemiconductor body is formed on an insulating substrate 402. Theinsulating substrate is comprised of a buried oxide or other insulatinglayer 406 over a silicon or other semiconductor substrate 404. A gatedielectric 416 is formed over the top and on the sides of thesemiconductor fin 410. A gate electrode 420 is formed over the top andon the sides of the gate dielectric. The gate electrode has a gatelength G_(L). The source, S, and drain, D, regions are formed in thesemiconductor fin on either side of the gate electrode.

The semiconductor fin has a top surface 412 and laterally oppositesidewalls 414. The semiconductor fin has a height or thickness equal toTsi. The semiconductor fin has a width equal to Wsi. The gate width of asingle fin tri-gate transistor is equal to the sum of the gate widths ofeach of the three gates formed on the semiconductor body, or,Tsi+Wsi+Tsi.

FIG. 5 illustrates a cross section of a typical dual fin tri-gatetransistor, 500, according to one embodiment of the present invention. Adual fin tri-gate transistor is a tri-gate transistor having twosemiconductor bodies, or fins, 410 over an insulating substrate 402, thetwo fins each having a gate dielectric formed on the top surface andlaterally opposite sidewalls, and sharing a single gate electrode formedover and around the gate dielectric. Each semiconductor fin has a topsurface 412 and laterally opposite sidewalls 414. The semiconductor finsare separated by a distance Ds. Patterning with normal lithographictechniques allows a minimum Ds of approximately 240 nm. The gate widthof a dual fin tri-gate transistor is equal to the sum of the gate widthsfor each of the two semiconductor bodies, or[2(Tsi1)+(Wsi1)]+[2(Tsi2)+(Wsi2)]. If the semiconductor bodies areformed in such a way that each semiconductor body has substantiallysimilar dimensions, the gate width of a dual fin tri-gate transistor iseffectively twice the gate width of a single fin tri-gate transistor.The gate width of a tri-gate transistor can be further increased byadding additional fins to the tri-gate transistor. A tri-gate transistorin a multiple fin configuration can deliver more drive current for agiven layout width than a planar transistor because a tri-gatetransistor having the same size as a planar transistor will have alarger gate width.

FIG. 6 illustrates a 6T CMOS SRAM cell circuit diagram using tri-gatetransistors according to one embodiment of the present invention. TheSRAM cell consists of two N-type access devices, 602, two N-typepull-down devices, 604, and two P-type pull-up devices, 606. Each of theN-type access devices 602 is a single fin tri-gate transistor. Each ofthe P-type pull-up devices 606 is a single fin tri-gate transistor. Eachof the N-type pull-down devices 604 is a dual fin tri-gate transistor.Using a dual fin tri-gate transistor as the pull-down device allows acircuit designer to achieve a higher cell ratio for the SRAM cell. Thedual fin tri-gate transistor will deliver more current than the singlefin tri-gate transistors, thus increasing the cell ratio withoutincreasing the cell layout size.

The cell ratio of an SRAM cell is defined as the ratio of thetransconductance factor of the pull-down N-type transistor to thetransconductance factor of the access N-type transistor. Thetransconductance factor of a transistor is equal to the ratio of gatewidth to gate length times the mobility and gate capacitance. Wheremobility and gate capacitance are constant across the access andpull-down transistors, the transconductance factor becomes the ratio ofthe transistor gate width to the transistor gate length. Thetransconductance factor of the dual fin tri-gate transistor will begreater than that of the planar transistor, because the transistor gatewidth of the dual fin tri-gate transistor is greater that of the planartransistor, within the same layout area. Furthermore, thetransconductance factor of the dual fin tri-gate transistor will begreater than that of the single fin tri-gate transistor because theratio of gate width to gate length for the dual fin device will begreater than that of the single fin device. Using a dual fin tri-gatetransistor as the pull-down device increases the transconductance factorof the pull-down device, thus increasing the cell ratio of the SRAMcell. As described above, higher, and thus more desirable static noisemargin (SNM) levels can be achieved by increasing the cell ratio. Theuse of non-planar tri-gate transistors in the design of an SRAM cellallows the cell ratio to be increased without increasing the physicalcell layout size. Table 1, below, is a comparison of cell ratios for anSRAM cell using planar transistors and an SRAM cell using tri-gatetransistors, where each of the SRAM cells has the same layout area.TABLE 1 Transistor Transconductance Planar SRAM Cell* Pull Down 1.6 CellRatio = 1.5 Access 1.1 Pull Up 1.3 Tri-gate SRAM Cell* Pull Down 6.0Cell Ratio = 2.15 Access 2.8 Pull Up 3.0*Note:Cell layout areas are the same for each cell

FIG. 7 illustrates a 6T CMOS SRAM cell layout using tri-gate transistorsaccording to one embodiment of the present invention. The gate of eachaccess device is located in region 702. The gate of each pull-downdevice is located in region 704. Each of the pull-down devices is a dualfin device. Each fin of the device is indicated by region 708 on eitherside of sacrificial block 709. Sacrificial block 709 is used to form thefins in close proximity to one another. The use of sacrificial block 709allows the fins to be spaced less than 100 nm from one another, whichwould not be possible using traditional lithography. The gate of eachpull-up device is located in region 706. The gate regions are indicatedby a region of polysilicon 714 over a region of P-type diffusion 712 orN-type diffusion 710. Metal layers 718 provide power (Vcc) and ground(Vss). Metal layers 718 may also connect the gate/source/drain of oneplanar transistor in the cell to the gate/source/drain of anothertransistor in the cell, and may connect one SRAM cell to another.Contacts 716 indicate regions where connections may be made to the metallayers. For a given Vcc, the cell ratio is tailored by sizing the gatewidth of each access transistor and each pull-down transistor to achievethe maximum SNM value. As described above, using an N-type dual fintri-gate device as the pull-down device and an N-type single fintri-gate device as the access device allows tri-gate SRAM cell to bedesigned having a higher cell ratio in the same layout area as a planarSRAM cell.

FIG. 8 is a comparison of the gate width of a dual fin tri-gatetransistor according to one embodiment of the present invention to thegate width of a planar transistor in the same layout area. Cross-section800 shows a dual fin tri-gate transistor formed on an insulatingsubstrate 808. The fins of the tri-gate transistor are formed bysemiconductor bodies 802. The fins are separated by a distance, Ds,which is determined by the width of the sacrificial block describedabove. The distance, Ds, may be defined by the minimum lithographyfeature size that may be patterned. A gate dielectric 804 covers eachfin of the tri-gate transistor in the gate region. Gate electrode 806 isformed over and around each semiconductor fin and gate dielectric layer.Three gates, G1, G2, and G3 are formed for each fin of the dual fintri-gate transistor. Each gate formed has a gate width. The gate widthof G1 is equal to Z1, or the height of the fin. The gate width of G2 isequal to Z2, or the width of the fin. The gate width of G3 is equal toZ3, or the height of the fin. The total gate width of each fin is equalto Z1+Z2+Z3. For a dual fin tri-gate transistor, the total gate width isequal to 2(Z1+Z2+Z3). A tri-gate transistor having N fins has a totalgate width equal to N(Z1+Z2+Z3). In one embodiment of the presentinvention, Z1=60 nm, Z2=60 nm, Z3=60 nm, and D_(s)=60 nm. The gate widthof the tri-gate transistor according to this embodiment is 2(60 nm+60nm+60 nm), or 360 nm. The total layout width used is equal toZ3+D_(s)+Z3, or (60 nm+60 nm+60 nm)=180 nm.

Cross-section 820 shows a planar transistor formed on a semiconductorsubstrate 828. The gate width of the planar transistor is equal to thewidth of transistor gate 822, or Zp. For a layout width of 180 nm, thegate width of planar transistor 820 is equal to 180 nm. Because the gatewidth of the tri-gate transistor is two times that of the planartransistor for the same layout area, it is possible to increase the cellratio of a 6T CMOS SRAM cell by designing the cell using single and dualfin tri-gate transistors according to one embodiment of the presentinvention.

FIG. 9 is a graph 900 which illustrates static noise margin (SNM) as afunction of VCC for a planar SRAM cell 920 and a tri-gate SRAM cell 910,where the cells are the same size. A tri-gate SRAM cell design allowsfor lower scaling of VCC before exceeding a lower SNM limit of 240 mV(930). Because the cell ratio is higher when the SRAM cell is designedusing tri-gate transistors according to one embodiment of the presentinvention, the supply voltage can be scaled lower without reducing theSNM below 240 mV. An SRAM cell designed using planar transistors can beoperated at a supply voltage of slightly less than 2.0V without reducingthe SNM to less than 240 mV. An SRAM cell of the same size, but designedusing dual and single fin tri-gate transistors according to oneembodiment of the present invention, may be operated at a much lowersupply voltage before the SNM limit is met. The supply voltage may be aslow as 1.25V before the SNM is reduced to less than 240 mV.

FIG. 10 is a flow diagram, 1000, showing a process in accordance withthe present invention, illustrating a general method for forming amultiple fin tri-gate transistor having a reduced layout width. Eachblock in flow diagram 1000 is illustrated and described in furtherdetail below, in conjunction with FIGS. 11A-11J.

A silicon or semiconductor film is formed on an insulating substrate, asdescribed in block 1002. The insulating substrate includes a lowermonocrystalline silicon substrate and a top insulating layer, such as asilicon dioxide film or a silicon nitride film. The insulating layer issometimes referred to as a “buried oxide” layer. In one embodiment ofthe present invention, the semiconductor film has a thickness of 60 nm.

A sacrificial block having a top surface and laterally oppositesidewalls is then formed on the semiconductor film, as described inblock 1004. In one embodiment of the present invention, the sacrificialblock is formed by first forming a layer of the sacrificial material andpatterning the sacrificial material to form a block using lithography.The sacrificial block may be comprised of nitride, but is not limited tonitride. The width of the sacrificial block determines the spacing ofthe fins. In one embodiment of the present invention, the laterallyopposite sidewalls of the sacrificial block are 60 nm apart. In anotherembodiment of the present invention, the laterally opposite sidewalls ofthe sacrificial block are separated by a distance defined by the minimumfeature size that may be formed using lithography.

After forming the sacrificial block, an insulating layer is formed overand around the sacrificial block and the semiconductor film, asdescribed in block 1006. The insulating layer may be comprised of anoxide or another insulating material. The insulating layer is depositedsuch that the thickness of the layer is approximately equal to thedesired semiconductor fin width. In one embodiment of the presentinvention, the thickness of the insulating layer is between 40 and 80nm. In another embodiment of the present invention, the thickness of theinsulating layer is 60 nm.

Insulating spacers are then formed on either side of the sacrificialblock by performing an anisotropic etch on the insulating layer, asdescribed in block 1008. After the anisotropic etch, insulating spacerswill remain on either side of the sacrificial block. The width of theinsulating spacers will be equal to the thickness of the originalinsulating layer. In one embodiment of the present invention, theinsulating spacers are 60 nm wide.

In another embodiment of the present invention, multiple sacrificialblocks may be formed, to form additional spacers. This method may beused to form a tri-gate transistor having more than two fins. The numberof fins formed will be equal to the number of insulating spacers. In oneembodiment of the present invention, an even number of fins (2N) may beformed. To form a tri-gate transistor having 2N fins, N sacrificialblocks and 2N insulating spacers are required.

After the insulating spacers are formed, the sacrificial block may beremoved by conventional methods, as shown in block 1010. For example, aselective etch process may be used to remove the sacrificial block,while the insulating spacers remain intact.

Next, two semiconductor fins are formed by etching the semiconductorfilm using the insulating spacers as a mask, as shown in block 1012. Thesemiconductor film is etched away in areas not covered by the insulatingspacers, exposing the insulating substrate. Each semiconductor finformed has a top surface and a pair of laterally opposite sidewalls.Using the insulating spacers as a mask allows the fins to be separatedby a distance that is less than the distance that could be achievedusing current lithographic technology. Current lithography allowsprinting of features having minimum sizes near 60 nm and minimum spacingbetween features of near 240 nm. Using an embodiment of a methodaccording to the present invention, the fins can be formed less than 240nm apart. In one embodiment of the present invention, the fins areseparated by a distance of 60 nm or less.

FIGS. 11A through 11J illustrate the formation of a dual fin tri-gatetransistor according to one embodiment of the present invention. Thefabrication of a dual fin tri-gate transistor begins with an insulatingsubstrate 1102, as shown in FIG. 11A. A silicon or semiconductor film1108 is formed on insulating substrate 1102. Insulating substrate 1102may be comprised of a lower monocrystalline silicon substrate 1104 and atop insulating layer 1106, such as a silicon dioxide or silicon nitridefilm. Insulating layer 1106 isolates semiconductor film 1108 fromsubstrate 1104, and is sometimes referred to as a “buried oxide” layer.Semiconductor film 1108 may be comprised of silicon or anothersemiconductor, such as but not limited to germanium (Ge), asilicon-germanium alloy (Si_(x)Ge_(y)), gallium arsenide (GaAs), InSb,GaP, GaSb, or carbon nanotubes. The semiconductor film 1108 may be anintrinsic, or undoped, silicon film, or it may be doped to a p-type orn-type conductivity. The semiconductor film 1108 is formed to athickness, Tsi, which is approximately equal to the height desired forthe subsequently formed semiconductor fins of the tri-gate transistor.In one embodiment of the present invention, the semiconductor film 1108has a thickness of 60 nm or less.

FIG. 11B illustrates the formation of a sacrificial block on the topsurface of semiconductor film 1108. The sacrificial block may be formedby conventional semiconductor manufacturing techniques, including butnot limited to depositing a layer of the sacrificial material, 1109, andsubsequently patterning the layer with resist 1111. The sacrificialmaterial that is not covered by resist 1111 may be etched to form one ormore sacrificial blocks in the desired location(s). In one embodiment ofthe present invention, the sacrificial material 1109 is comprised of anitride. The width of the sacrificial block to be formed, Ws, willdefine the subsequent spacing of the semiconductor fins of the tri-gatetransistor. In one embodiment of the present invention, Ws is 60 nm orless. The use of a sacrificial block allows the semiconductor fins to beseparated by distances of 60 nm or less, which is significantly lessthan the distance between features that can be achieved by conventionallithography techniques.

FIG. 11C illustrates the formation of an insulating layer 1112 over andaround the insulating block 1110 and over the surface of thesemiconductor film 1108. In one embodiment of the present invention, theinsulating layer is comprised of an oxide. The insulating layer 1112 isdeposited in a manner which allows the layer to have a uniformthickness, Tox. The thickness of the insulating layer will determine thewidth of the semiconductor fins in a subsequent processing step. In oneembodiment of the present invention, the insulating layer has athickness of 60 nm or less.

FIG. 11D illustrates the formation of insulating spacers 1114.Insulating spacers 1114 are formed by performing an anisotropic etch onthe insulating layer 1112 of FIG. 11C. The anisotropic etch is performedin a manner that allows the insulating layer to be entirely removed fromthe top surface of the sacrificial block 1110, but leaves insulatingspacers 1114 on either side of the sacrificial block. The insulatingspacers 1114 are formed having a width, Wox, which is equal to thethickness of the insulating film of FIG. 11C, Tox. In one embodiment ofthe present invention, the width, Wox, of each insulating spacer is 60nm or less.

FIG. 11E illustrates the structure formed after the removal of thesacrificial block. The sacrificial block may be removed by conventionalmethods, including the use of a selective etch process. For example, awet etch may be used to remove a sacrificial nitride block, while theoxide spacers will remain unaffected by the etch process. After thesacrificial block is removed, two insulating spacers 1114 remain, eachspacer having a width equal to Wox. The spacers are separated by adistance equal to the width of the sacrificial block, Ws.

FIG. 11F illustrates the formation of semiconductor fins 1120.Semiconductor fins 1120 are formed by etching semiconductor film 1108using insulating spacers 1114 as a mask. In one embodiment of thepresent invention, the etch is a plasma dry etch process. Thesemiconductor film is etched completely, exposing the surface ofinsulating substrate 1102. The semiconductor fins are formed having awidth, Wsi, which is equal to the width of the insulating spacers usedas a mask. In one embodiment of the present invention, Wsi is 60 nm orless. The semiconductor fins are separated by a distance, Ds, which isequal to the width of the sacrificial block formed previously. In oneembodiment of the present invention, Ds is 60 nm or less.

After the semiconductor fins 1120 are formed, the insulating spacers maybe removed by conventional techniques, as illustrated in FIG. 11G. Atthis point, two semiconductor fins 1120 remain on the insulatingsubstrate 1102. The semiconductor fins 1120 have a top surface, 1121,and laterally opposite sidewalls, 1123. The total layout width of thedevice will be equal to Wsi+Ds+Wsi. In one embodiment of the presentinvention, the total layout width of the device is 180 nm or less.

FIG. 11H illustrates the formation of a gate dielectric layer 1122 onthe top surface 1121 and on the sidewalls 1123 of each semiconductor fin1120. The tri-gate transistor can be designed to be inherently immunefrom Vt instability by careful control over the geometry of the corners1125 of the semiconductor fin. The corner of the semiconductor fin isformed by the intersection of adjacent gates, G1, G2, and G3, (top andside) of the device. Because the corner 1125 of the tri-gate transistorturns on first, it determines the threshold voltage (Vt) of the device.When Vt is set only by dopant implants, there may be fluctuations in thedopants which in turn can cause Vt fluctuations. When corner rounding iscontrolled, the tri-gate transistor is not dependent on doping to setthe Vt, and so the transistor can be designed to be inherently immunefrom Vt instability. Corner rounding of the silicon fin arises primarilyfrom the gate dielectric formation process. The gate dielectric 1122 maybe grown or deposited on the surface and sidewalls of the silicon fin.In one embodiment of the present invention, the gate dielectric layer isdeposited using Atomic Layer Deposition (ALD), which allows cornerrounding to be controlled to atomic dimensions. In one embodiment of thepresent invention, the radius of curvature, R, of each corner of thesemiconductor fin is less than 10 nm.

Next, a gate material is deposited over the top surface and sidewalls ofeach semiconductor fin and over the insulating substrate, as illustratedin FIG. 11I. The gate material is patterned to form a gate electrode1124 on the gate dielectric layer.

After the gate electrode is formed, a pair of source/drain regions isformed in each semiconductor fin on opposite sides of the gateelectrode, as illustrated in FIG. 11J. In one embodiment of the presentinvention, source and drain regions are formed by implanting N-type orP-type dopants into the semiconductor body, as illustrated by arrows1130. In embodiments of the present invention, further operations may beperformed on the tri-gate device, including, but not limited to,formation of tip or source/drain extension regions, halo regions,heavily doped source/drain contact regions, silicon deposited on thesource/drain and gate electrode regions, and silicide formation on thesource/drain and gate electrode regions.

As illustrated in FIG. 11J, each semiconductor fin of the resulting dualfin tri-gate transistor has a gate width that is equal to 2Tsi+Wsi. Thegate width for the dual fin tri-gate transistor is equal to the sum ofthe gate widths for each fin, or 2(2Tsi+Wsi). The device may bemanufactured in an area having a layout width of 2Wsi+Ds. In oneembodiment of the present invention, the gate width of the dual fintri-gate transistor is 360 nm or less, and the device is formed in anarea having a layout width of 180 nm or less.

In other embodiments of the present invention, the method set forthabove may be used to form tri-gate transistors having more than twosemiconductor fins.

1. A method of forming a six transistor (6T) CMOS SRAM cell, comprising:forming two N-type access devices, each N-type access device comprisedof a tri-gate transistor having a single fin; forming two P-type pull-updevices, each P-type pull-up device comprised of a tri-gate transistorhaving a single fin; forming two N-type pull-down devices, each N-typepull-down device comprised of a tri-gate transistor having at least twofins.